examples/scripts.noah/require.bash

   1 # require.bash
   2 # Author: Noah Friedman <friedman@prep.ai.mit.edu>
   3 # Created: 1992-07-08
   4 # Last modified: 1993-09-29
   5 # Public domain
   6
   7 # Commentary:
   8
   9 # These functions provide an interface based on the lisp implementation for
  10 # loading libraries when they are needed and eliminating redundant loading.
  11 # The basic idea is that each "package" (or set of routines, even if it is
  12 # only one function) registers itself with a symbol that marks a "feature"
  13 # as being "provided".  If later you "require" a given feature, you save
  14 # yourself the trouble of explicitly loading it again.
  15 #
  16 # At the bottom of each package, put a "provide foobar", so when another
  17 # package has a "require foobar", it gets loaded and registered as a
  18 # "feature" that won't need to get loaded again.  (See warning below for
  19 # reasons why provide should be put at the end.)
  20 #
  21 # The list of provided features are kept in the `FEATURES' variable, which
  22 # is not exported.  Care should be taken not to munge this in the shell.
  23 # The search path comes from a colon-separated `FPATH' variable.  It has no
  24 # default value and must be set by the user.
  25 #
  26 # Require uses `fpath_search', which works by scanning all of FPATH for a
  27 # file named the same as the required symbol but with a `.bash' appended to
  28 # the name.  If that is found, it is loaded.  If it is not, FPATH is
  29 # searched again for a file name the same as the feature (i.e. without any
  30 # extension).  Fpath_search may be useful for doing library filename
  31 # lookups in other functions (such as a `load' or `autoload' function).
  32 #
  33 # Warning: Because require ultimately uses the builtin `source' command to
  34 # read in files, it has no way of undoing the commands contained in the
  35 # file if there is an error or if no provide statement appeared (this
  36 # differs from the lisp implementation of require, which normally undoes
  37 # most of the forms that were loaded if the require fails).  Therefore, to
  38 # minize the number of problems caused by requiring a faulty package (such
  39 # as syntax errors in the source file) it is better to put the provide at
  40 # the end of the file, rather than at the beginning.
  41
  42 # Code:
  43
  44 # Exporting this variable would cause considerable lossage, since none of
  45 # the functions are exported (or at least, they're not guaranteed to be)
  46 export -n FEATURES
  47
  48 #:docstring \f:
  49 # Null function.  Provided only so that one can put page breaks in source
  50 # files without any ill effects.
  51 #:end docstring:
  52 #
  53 # (\\014 == C-l)
  54 eval "function $(echo -e \\014) () { : }"
  55
  56
  57 #:docstring featurep:
  58 # Usage: featurep argument
  59 #
  60 # Returns 0 (true) if argument is a provided feature.  Returns 1 (false)
  61 # otherwise.
  62 #:end docstring:
  63
  64 ###;;;autoload
  65 function featurep ()
  66 {
  67     local feature="$1"
  68
  69     case " ${FEATURES} " in
  70        *" ${feature} "* ) return 0 ;;
  71     esac
  72
  73     return 1
  74 }
  75
  76
  77 #:docstring provide:
  78 # Usage: provide symbol ...
  79 #
  80 # Register a list of symbols as provided features
  81 #:end docstring:
  82
  83 ###;;;autoload
  84 function provide ()
  85 {
  86     local feature
  87
  88     for feature in "$@" ; do
  89        if ! featurep "${feature}" ; then
  90           FEATURES="${FEATURES} ${feature}"
  91        fi
  92     done
  93
  94     return 0
  95 }
  96
  97
  98 #:docstring require:
  99 # Usage: require feature {file}
 100 #
 101 # Load FEATURE if it is not already provided.  Note that require does not
 102 # call `provide' to register features.  The loaded file must do that
 103 # itself.  If the package does not explicitly do a `provide' after being
 104 # loaded, require will complain about the feature not being provided on
 105 # stderr.
 106 #
 107 # Optional argument FILE means to try to load FEATURE from FILE.  If no
 108 # file argument is given, require searches through FPATH (see fpath_search)
 109 # for the appropriate file.
 110 #
 111 # If the variable REQUIRE_FAILURE_FATAL is set, require will cause the
 112 # current shell invocation to exit, rather than merely return.  This may be
 113 # useful for a shell script that vitally depends on a package.
 114 #
 115 #:end docstring:
 116
 117 ###;;;autoload
 118 function require ()
 119 {
 120  local feature="$1"
 121  local path="$2"
 122  local file
 123
 124    if ! featurep "${feature}" ; then
 125       file=$(fpath_search "${feature}" "${path}") && source "${file}"
 126
 127       if ! featurep "${feature}" ; then
 128          echo "require: ${feature}: feature was not provided." 1>&2
 129          if [ "${REQUIRE_FAILURE_FATAL+set}" = "set" ]; then
 130             exit 1
 131          fi
 132          return 1
 133       fi
 134    fi
 135
 136    return 0
 137 }
 138
 139 #:docstring fpath_search:
 140 # Usage: fpath_search filename {path ...}
 141 #
 142 # Search $FPATH for `filename' or, if `path' (a list) is specified, search
 143 # those directories instead of $FPATH.  First the path is searched for an
 144 # occurrence of `filename.bash, then a second search is made for just
 145 # `filename'.
 146 #:end docstring:
 147
 148 ###;;;autoload
 149 function fpath_search ()
 150 {
 151  local name="$1"
 152  local path="$2"
 153  local suffix=".bash"
 154  local file
 155
 156     if [ -z "${path}" ]; then path="${FPATH}"; fi
 157
 158     for file in "${name}${suffix}" "${name}" ; do
 159        set -- $(IFS=':'
 160                  set -- ${path}
 161                  for p in "$@" ; do
 162                     echo -n "${p:-.} "
 163                  done)
 164
 165        while [ $# -ne 0 ]; do
 166           test -f "${1}/${file}" && { file="${1}/${file}"; break 2 }
 167           shift
 168        done
 169     done
 170
 171     if [ $# -eq 0 ]; then
 172        echo "fpath_search: ${name}: file not found in fpath" 1>&2
 173        return 1
 174     fi
 175
 176     echo "${file}"
 177     return 0
 178 }
 179
 180 provide require
 181
 182 # require.bash ends here